1. Field of the Invention
The present invention relates to photographic apparatus for self-developing, peel-apart film, in general, and to a film cassette for an automatic film processor enclosing a plurality of self-developing, peel-apart film unit elements of the type having their leading ends adapted for engagement by a force-transmitting member for movement into or out of the film cassette for subsequent film unit processing, in particular.
2. Description of the Prior Art
Self-developing, peel-apart film units are well known in the field of instant photography. Each film unit comprises a negative or photosensitive sheet for forming a negative image of a subject, an image receiving sheet for forming a positive subject image and a rupturable pod of processing liquid. A positive images is formed by means of a well-known diffusion transfer process after the pod containing the processing liquid is ruptured and its contents spread between the positive and negative film sheets.
Film units of the aforementioned type are typically exposed and processed in portable, multiple-exposure, instant-type photographic cameras or processed in large format film processing equipment. When employed in a camera as many as ten film units are provided in a single lighttight film negative sheet on the other side of a pressure plate located within the film pack. Pull tabs are attached to one or both of the leading ends of the positive and negative sheets. After exposing an outermost negative sheet at a film plane of the camera a first tab attached to the negative sheet is withdrawn from the camera forcing the exposed negative to be turned 180.degree. about the pressure plate and into superposition with the positive sheet. This movement causes a second tab attached to the forward ends of the positive and negative sheets to enter and pass through the bite of a pair of motor driven pressure applying spread rollers to a position exterior of the camera. A combination of the angle at which the first tab is attached and the force applied by a camera operator causes the first tab to detach from the negative sheet at this time.
After the first tab is so detached, the operator pulls the second tab so as to advance the positive and negative sheets between the pair of spread rollers in a superposed relation. At the beginning of the advancement of the positive and negative sheets the spread rollers apply pressure to a pod containing a processing liquid located at the leading ends thereof, thereby rupturing the same, and spreading its contents between the superposed sheets in a thin uniform layer as the sheets are advanced therebetween. A positive image is formed on the positive sheet outside of the camera by a well known diffusion transfer process. After a required interval of time the positive sheet is peeled away from the negative sheet and the negative sheet, along with the second tab attached thereto are discarded. A more detailed description of this particular type of peel-apart film may be found at pages 185-187 of a book entitled, "Camera Technology" by Norman Goldberg published in 1992 by the Academic Press.
The above-described peel-apart film format has several disadvantages, especially if considered for use in certain photographic apparatus. One disadvantage is film cost. In addition to the multiple tabs and the relatively large amounts of leader material required for each film unit, which necessarily increases material costs, economies of scale are limited by the fact that both a positive and negative sheet must be included within a single film cassette in order to take advantage of such economies, which would substantially increase cassette size and thereby make such a cassette unwieldy for use by a camera operator. Another disadvantage is that this type of film format is not susceptible of inclusion in highly automated film processing equipment. Processing a film unit by pulling certain tabs at various times in the film processing cycle is clearly a processing technique that is inherently manual.
In U.S. patent application Ser. No. 936,513 filed Aug. 28, 1992 by Polizzotto et al., and assigned to the assignee of the present application, an improved film format for a self-developing, peel-apart film unit is disclosed that was specifically designed for use in highly automated photographic apparatus utilizing peel-apart film. A film unit 10 incorporating this improved film format is shown in drawing FIGS. 1A and 1B. The film unit comprises both positive and negative sheet elements with each having pockets formed on a surface of a leading end thereof. A force-transmitting member (or members) engage these sheet element pockets and move the so engaged sheet elements, in a mated configuration, into the bite of a pair of spread rollers in a conventional developer liquid spread system of an automatic film processor in order to initiate the film developing process.
A major problem associated with a photographic sheet element transport system of this type is the inability to consistently insert a force-transmitting member into a sheet element pocket and thereby avoid a sheet element misfeed into, for example, the developer liquid spread system. If the force-transmitting member fails to enter a sheet element pocket formed on a surface thereof, sheet element movement into the developer liquid spread system cannot occur. A correlative problem is the inability to consistently withdraw the force-transmitting member from the sheet element pocket without damaging a sheet element surface. If the path of force-transmitting member movement is not controlled while being withdrawn from the sheet element pocket for movement back toward its start or initial position, it may be dragged across and thereby scratch a sheet element surface. The photosensitive layer of the negative sheet element of a self-developing photographic film unit would be particularly susceptible to such force-transmitting member movement damage.